Libertas is, without doubt, 'freedom' in the sense of liberty(liber is the adjective 'free', 'not a slave' etc)
Obliviscor (I forget) is a, sadly, the normal verb for 'forget'. Addressing more than one person we, strictly, need the the imperative plural which is:
obliviscimini
So our 'Never forget' is the ugly " numquam obliviscimini!" [or 'nunquam': nunquam and nunquam are both correct, nunquam being an earlier form of numquam]
However, QM has it better:
obliviscere is the imperative singular ,so 'nunquam/ numquam obliviscere' is spot on . Now, the Romans would surely have preferred the singular and written 'nunquam obliviscere' on monuments, because a) the plural is so ugly and b) they'd allow themselves the conceit that they were addressing one person, the reader, and not lots of people at one time.